It is well-known to use information on the directions to sound sources in relation to a listener for distinguishing between noise sources and desired sound sources. Throughout the present specification, the term directional signal processing system means a signal processing system that is adapted to exploit the spatial properties of an acoustic environment. Directional microphones are available, but typically directional signal processing systems utilize an array of omni-directional microphones.
The directional signal processing system combines the electrical signals from the microphones in the array into a signal with varying sensitivity to sound sources in different directions in relation to the array. Throughout the present specification, a plot of the varying sensitivity as a function of the direction is denoted the directivity pattern. Typically, a directivity pattern has at least one direction wherein the microphone signals substantially cancel each other. Throughout the present specification, such a direction is denoted a null direction. A directivity pattern may comprise several null directions depending on the number of microphones in the array and depending on the signal processing.
Directional signal processing systems are known that prevent sound suppression of sources in certain directions of interest.
For example, U.S. Pat. No. 5,473,701 discloses a method of enhancing the signal-to-noise ratio of a microphone array with an adjustable directivity pattern, i.e. an adjustable null direction, for reduction of the microphone array output signal level in accordance with a criterion wherein the reduction is performed under a constraint that the null direction is precluded from being located within a predetermined region of space.
It is an object to provide a system with an improved capability of suppressing sound sources from all directions.